Final answer:
The basic Keynesian model assumes that firms meet demand at preset prices, which are often 'sticky' and don't adjust quickly to changes in the economic climate.
Step-by-step explanation:
The key assumption of the basic Keynesian model is b. firms meet demand at preset prices. This is linked to one of the central ideas in Keynesian economics, which suggests that prices and wages can be 'sticky' — that is, they do not adjust immediately to changes in economic conditions. Instead, businesses meet current demand at existing prices even if they are not in equilibrium. This can lead to excess supply or demand in the short term, potentially resulting in unemployment or inflation. Additionally, the Keynesian view posits that aggregate demand is often the driving force behind economic fluctuations, particularly in the short run.